It is known, for example from U.S. Pat. No. 4,893,649, to provide a fluid flow regulator in which a spring biased piston is disposed between two chambers of which a first is connected to an inlet and a second is connected to an outlet by way of a valve of which a closure member, such as a valve needle, is carried, typically with an intermediate spring, by the piston, there being a flow passage between the first and second chambers. The piston responds to a pressure difference between the chambers so that it tends to close the valve in response to a relative increase in pressure at the inlet. Adjustment of the flow rate may be achieved by positional adjustment of the valve seat. Flow regulators of this general type regulate the flow by means of a spring-bias or load that balances the pressure difference. A change in the pressure difference is counteracted by a change in the spring-load. The flow rate varies as the square root of the pressure difference across a fixed orifice.
As will be explained more particularly later, a difficulty in the design of flow regulators which need to provide continuous flow at very low rates (particularly for chemical injection sub-sea) is conflict between a need for a small control orifice and avoidance of clogging by particles, as well as the provision of a substantial ‘turn-down ratio’ i.e. the ratio of the maximum to the minimum flow rate. An object of the invention is to provide an improved regulator in this context.